In recent years, there is a strong demand for a change in film substrates of flat panel displays, which are typified by liquid crystal and organic EL displays, and solar cell panels to be thinner, lighter and more flexible than current glass or silicon inorganic material substrates, from the viewpoint of energy conservation and environment conservation.
A liquid crystal display panel is constituted by joining together in a multilayered manner a multitude of functional films with a light management function, in addition to a liquid crystal enclosed between two glass substrates. Examples of such functional films are polarizing films, phase difference films, antireflective films, light control films, protective films, and so on.
In the manufacturing process of the above-described liquid crystal display panel, a joining process is frequently performed in which different types of films made of different materials or having different properties are joined, or in which a glass and a film are joined. However, in general, an organic material film has lower heat resistance than an inorganic material substrate. Thus, a heat process of several hundred degrees is not applicable to the manufacturing process. Accordingly, methods such as application of an adhesive and thermal compression bonding at relatively low temperature are used.
As a conventional technique of joining films together using an adhesive, for example, a technique is disclosed in which a thermoplastic saturated norbornene resin, which received a corona discharge treatment, is joined to a polarizer made of polyvinyl alcohol via a polyester polyol adhesive layer, thereby to form a protection film of a polarizing plate (see Patent Document 1).
Other technique is also disclosed in which a cycloolefin resin film is layered via an adhesive layer on one surface of a polarizer (polarizing film) and a cellulose acetate resin film is layered via an adhesive layer on the other surface of the polarizer, thereby to form a polarizing plate (see Patent Document 2).
On the other hand, a technique is also proposed in which a protection film and a polarizer are joined together without an adhesive therebetween. For example, a technique is disclosed in which a protection film is formed with two layers of resin films having different softening points, and joined to a polarizer on a side of the layer having a lower softening point for thermal compression bonding, thereby to obtain a polarizing plate (see Patent Documents 3 to 5). Also disclosed is a method of manufacturing a polarizing plate by joining a protection film and a polarizer together by heating at a temperature lower than the softening point of the protection film or by irradiation of electromagnetic waves (see Patent Document 6).